1. Field of the Invention
This invention relates generally to the processing of electronic article surveillance (EAS) tag signals, and more particularly to a system and method of using phase shifting of a plurality of transmitter oscillators in a transmitter used in an EAS system.
2. Description of the Related Art
In acoustomagnetic or magnetomechanical electronic article surveillance, or “EAS,” a detection system may excite an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. When the tag is present within an interrogation zone defined by the electromagnetic field generated by the burst transmitter, the tag resonates with an acoustomagnetic or magnetomechanical response frequency that is detectable by a receiver in the detection system.
The typical default mode of operation of these EAS systems in most countries that do not adhere to the standards promulgated by the European Telecommunications Standards Institute (“ETSI”) uses phase flipping on the transmitter to produce various electromagnetic field patterns that provide for excitation of the tags in various orientations. However, the emissions standards in some countries (notably those adhering to ETSI standards) prevent the system from transmitting in certain antenna configurations with any significant current levels.
For example, a figure eight antenna configuration produces an electromagnetic field that meets ETSI standards, but tags located in certain positions and orientations within the interrogation zone may not get excited by the figure eight antenna configuration because these tags are located in “nulls” within the resultant electromagnetic field. An aiding antenna configuration produces fewer nulls, but particular current levels may result in electromagnetic field levels that do not meet the ETSI standards. Another issue is that due to mismatches in the antenna tuning, there may be phase shifts between the two antenna elements. These mismatches result in an imperfect electromagnetic field, for example, decreased power efficiency in the interrogation zone and increased emission levels in figure eight antenna configurations. Decreased power efficiency makes the excitation and subsequent detection of EAS tags within the interrogation zone more difficult. Increased emission levels may not meet ETSI standards.